Process for the bleaching of mechanical paper pulp

ABSTRACT

The invention relates to a process for the bleaching of paper pulp with one or more peroxide oxidizing agents, in which the pulp is first treated in a pretreatment step with at least one inorganic magnesium derivative combined with at least one carboxylic acid or its salt, except its magnesium salt, or with at least one carboxylic acid at least partially saponified with Mg 2+ , the carboxylic acid being substituted by at least one electrodonating group or comprising at least one carbon-carbon double bond.

The present application claims the benefit of the European application no. 09161461.0 filed on May 29, 2009, herein incorporated by reference.

The present invention is related to a process for the bleaching of paper pulp with one or more peroxide oxidizing agents, in which the paper pulp is pretreated with a chelating agent.

It is known to bleach paper pulp with peroxide oxidizing agents such as peracids or hydrogen peroxide.

It is also known to proceed to a pretreatment step of the paper pulp with a chelating agent, prior to the bleaching step, in order to selectively eliminate metallic ions detrimental to bleaching. Indeed, some metallic ions catalyze decomposition reactions of the peroxide compounds. The most detrimental ions are manganese, iron, and copper.

Widely used chelating agents are those of the aminocarboxylic type, such as ethylene diamine tetra-acetic acid and its salts (EDTA) or diethylene triamine pentaacetic acid and its salts (DTPA). Nevertheless, these chelating agents are poorly biodegradable and will thus be released in the environment where they will accumulate. Although their toxicity is not really an issue, their accumulation in lakes and rivers will lead to the mobilization of heavy metals that are present in the soil and that will become soluble in water. These heavy metals will then be assimilated by living organisms like fishes, therefore entering in the food chain.

The use of biodegradable chelating agents such as aspartic acid and its derivatives, substituted on the nitrogen, is described in the international application WO 97/30209. Such products include ethylenediamine-N,N′-disuccinic acid (EDDS) and 2,2′-imino-disuccinic acid (IDS). According to that previous application, the pretreatment step must be conducted at a pH from 4 to 8 while EDDS and IDS perform best at a high pH, especially pH 8 for EDDS and 10 for IDS. Thus, this process does not lead to an optimal brightness of the paper pulp.

The international application WO 99/46441 describes a process for the bleaching of mechanical paper pulp with one or more peroxide oxidizing agents in which the pulp is pretreated with one or more chelating agents chosen from aspartic acid and its derivatives, substituted on the nitrogen, at a pH above 8. The natural pH of the paper pulp being usually between 5 and 6, such process implies the measurement and the control of the pH during the whole pretreatment step, by the addition of a base such as caustic soda. This has the drawback of lowering the brightness of the paper pulp fed to the subsequent bleaching step, due to a yellowing effect of the paper pulp during the pretreatment step. Furthermore, the addition of caustic soda during the pretreatment step increases the amount of extractives leading to a reduced yield in pulp and a higher chemical oxygen demand (COD) charge in the effluent treatment plant.

A further drawback of the use of EDDS and IDS as chelating agents is the higher operating costs resulting from the higher price or the higher concentration required and of the additional use of a base to maintain the pH above 8 during the pretreatment step.

The purpose of the present invention is to provide a simple and environmentally friendly bleaching process, avoiding the measurement and control of the pH during the pretreatment step and still conducting to an optimal brightness of the paper pulp.

The present invention therefore relates to a process for the bleaching of paper pulp with one or more peroxide oxidizing agents, in which the pulp is first treated in a pretreatment step with one or more chelating agents chosen from the group consisting of:

-   (i) carboxylic acids or their salts, except their magnesium salt,     substituted by at least one electrodonating group or comprising at     least one carbon-carbon double bond, in the presence of at least one     inorganic magnesium derivative, and -   (ii) carboxylic acids at least partially saponified with Mg²⁺ and     substituted by at least one electrodonating group or comprising at     least one carbon-carbon double bond.

Indeed, it has surprisingly been found that it is possible to reach a high degree of brightness or to decrease the hydrogen peroxide consumption for a given brightness or both, when the pretreatment step is conducted in the presence of at least one inorganic magnesium derivative in combination with at least one carboxylic acid or its salt (i) or in the presence of a magnesium salt of a carboxylic acid (ii), the carboxylic acid being substituted by at least one electrodonating group or comprising at least one carbon-carbon double bond.

One of the essential features of the present invention resides in the use, during the pretreatment step, of an inorganic magnesium derivative combined with a carboxylic acid or its salt (i), or of a magnesium salt of a carboxylic acid (ii), the carboxylic acid being substituted by at least one electrodonating group or comprising at least one carbon-carbon double bond.

According to the present invention, the carboxylic acids or the salts thereof, useful as chelating agents during the pretreatment step, are usually substituted by at least one electrodonating group and/or comprise at least one carbon-carbon double bond within the skeleton of the molecule. Suitable electrodonating group are usually selected from the group consisting of amines, especially alkylamines or dialkylamines, alcohols, thiols, O-alkyl and 5-alkyl. Preferred electrodonating groups are the amino group (—NH₂), the alkylamino group (—NRH) or the dialkylamino group (—NRR′) or the hydroxyl group (—OH), in particular the dialkylamino group (—NRR′) or the hydroxyl group (—OH).

In the present process, suitable carboxylic acids are those of the following general formula (I)

HOOC—C_(n)H_(m)(OR¹)_(p)(SR²)_(q)(NR³R⁴)_(r)(COOH)_(s)—R′  (I)

wherein C_(n)H_(m) is linear or branched and saturated or unsaturated; n is an integer from 1 to 8, especially from 1 to 4; m is an integer from 0 to 2n; p is an integer from 0 to n; q is an integer from 0 to n; r is an integer from 0 to n; s is an integer from 0 to n; p+q+r+s is an integer from 0 to 2n, preferably at most n, and if p+q+r+s is 0, m is ≦2n−2; R′ is selected from —H, —CH₂OH, —COOH; and R¹, R², R³, R⁴ are the same or different and are selected from —H and linear or branched groups of formula —C_(n′)H_(m′)—R″, wherein n′ is an integer from 1 to 8, m′ is an integer from 0 to 2n and R″ has the same definition as R′ above; preferably R¹, R², R³, R⁴ are selected from —H, —CH₂—CH₂OH, —CH₂—COOH and —CH(CH₃)—COOH.

The carboxylic acids used in the process of the present invention are typically linear. They can also be branched. Usually they are staturated. They can also be unsaturated. Preferably, the carboxylic acids are not cyclic. In particular, they are not aromatic compounds.

In the process of the invention, the carboxylic acids or the salts thereof comprise at least one carboxylic acid or carboxylate group, preferably at least two carboxylic acid or carboxylate groups, for example two, three or even four carboxylic acid or carboxylate groups.

Especially suitable carboxylic acids are those of the formula (II), (III), (IV) or (V) below:

HOOC—C_(n)H_(m)—COOH  (II)

wherein n is an integer from 2 to 8, preferably from 2 to 4 and m is an integer from 0 to 2n−2, preferably from n to 2n−2;

HOOC—C_(n)H_(m)(OH)_(p)—COOH  (III)

wherein n is an integer from 2 to 8, preferably from 2 to 4, and m and p=n;

HOOC—C_(n)H_(m)(OH)_(p)(COOH)_(s)—COOH  (IV)

wherein n is an integer from 2 to 8, preferably from 2 to 4, and m is an integer from 0 to 2n−p−s, p is an integer from 1 to n, s is an integer from 1 to n; and

HOOC—C_(n)H_(m)(NR¹R²)—H  (V)

wherein n is an integer from 1 to 8, preferably from 1 to 4, in particular 1 or 2, m is an integer from 1 to 2n−1, R¹ and R² are the same or different and are selected from —H, —CH₂—CH₂OH, —CH₂—COOH and —CH(CH₃)—COOH, in particular from —CH₂—COOH and —CH(CH₃)—COOH.

Especially suitable examples of carboxylic acids, or salts thereof, useful in the present invention are lactic acid, citric acid, tartaric acid, glucaric acid, maleic acid, nitrilotriacetic acid (NTA), methylglycine diacetic acid (MGDA), their salts and mixtures thereof. Citric acid, maleic acid, nitrilotriacetic acid (NTA), methylglycine diacetic acid (MGDA), their salts and mixtures thereof are preferred. According to the present invention, the most preferred carboxylic acids, or salts thereof, are citric acid, nitrilotriacetic acid (NTA), methylglycine diacetic acid (MGDA), their salts and mixtures thereof.

In the process of the invention, good results are obtained using only one carboxylic acid or salt thereof, substituted by at least one electrodonating group and/or comprise at least one carbon-carbon double bond within the skeleton of the molecule. Nevertheless, more than one carboxylic acid or salt thereof according to the invention can be used. For example, several carboxylic acids and/or salts thereof according to the invention can be combined. A carboxylic acid or a salt thereof according to the invention can also be combined with one or more conventional chelating agents such as ethylene diamine tetra-acetic acid and its salts (EDTA) or diethylene triamine pentaacetic acid and its salts (DTPA).

According to the present invention, preferably, there is no peroxide oxidizing agent present during the pretreatment step of the paper pulp with the chelating agents.

In the present invention, it is preferable to select carboxylic acids which are biodegradable products. The preferred carboxylic acids according the present invention as well as their salts have the main advantage to be readily biodegradable products.

In a first embodiment of the present invention, the pretreatment step is conducted in the presence of at least one inorganic magnesium derivative and of at least one carboxylic acid or its salt, except its magnesium salt (i), as defined above, namely substituted by at least one electrodonating group and/or comprising at least one carbon-carbon double bond within the skeleton of the molecule.

The salt of the at least one carboxylic acid may be selected from, for example, the sodium salt, the potassium salt, the lithium salt of the carboxylic acid.

According to this first embodiment, the inorganic magnesium derivative is preferably selected from magnesium oxide, magnesium hydroxide, magnesium chloride, magnesium nitrate and magnesium sulphate. Magnesium sulphate may be in its anhydrous form or in its hydrated form, for example in its heptahydrated form (Epsom salt). An especially preferred inorganic magnesium derivative according to this first embodiment is the magnesium sulphate heptahydrate or Epsom salt.

In this first embodiment, the amount of the inorganic magnesium derivative must be optimized depending on the paper pulp to be pretreated and on the paper mill. The amount of the inorganic magnesium derivative, expressed as Mg²⁺ ions, is usually less than or equal to 1% by weight of the dry paper pulp, preferably less than or equal to 0.5%, more preferably lower than or equal to 0.1%, advantageously less than or equal to 0.08%. The amount of the inorganic magnesium derivative, expressed as Mg²⁺ ions, is generally more than or equal to 0.01% by weight of the dry paper pulp, preferably more than or equal to 0.02%, advantageously more than or equal to 0.04%.

Still according to this first embodiment, the concentration of the carboxylic acid or its salt (i) is typically less than or equal to 1% by weight of the dry pulp, preferably less than or equal to 0.8% of dry pulp. The concentration of the carboxylic acid or its salt (i) is usually more than or equal to 0.2% by weight of the dry pulp, preferably more than or equal to 0.3% by weight of dry pulp, for example more than or equal to 0.4% by weight of dry pulp.

In a second embodiment of the present invention, the pretreatment step is conducted in the presence of at least one carboxylic acid at least partially saponified with Mg²⁺ (ii) as defined above, namely substituted by at least one electrodonating group and/or comprising at least one carbon-carbon double bond within the skeleton of the molecule. This second embodiment is especially advantageous in the process of the invention. Said magnesium salt of carboxylic acid can be purchased as such to an industrial manufacturer or can be prepared before use by reacting a magnesium derivative such as magnesium oxide, magnesium hydroxide or magnesium carbonate, with the corresponding carboxylic acid to form the at least partial magnesium salt of the carboxylic acid, according to any method known in the art.

According to this second embodiment, the at least partial magnesium salt of the carboxylic acid (ii) is generally less than or equal to 1% by weight of the dry pulp, especially less than or equal to 0.8%. The concentration of the magnesium salt of the at least partial magnesium salt of the carboxylic acid (ii) is in general more than or equal to 0.2% by weight of the dry pulp, preferably more than or equal to 0.4%.

The at least partial magnesium salt of the carboxylic acid (ii) may be in the form of the stoechiometric magnesium salt or of a partial magnesium salt. Indeed, all the carboxylic groups of all the carboxylic acid molecules may be saponified, or only a part of the carboxylic groups and/or a part of the carboxylic acid molecules may be saponified.

Examples of stoechiometric salts of the carboxylic acids of the present invention are magnesium citrate, magnesium maleate and the magnesium salt of nitrilotriacetic acid. Magnesium citrate results from the stoechiometric reaction of two equivalents of citric acid with 3 equivalents of Mg(OH)₂ and can be written as Citrate₂Mg₃. Magnesium maleate results from the stoechiometric reaction of one equivalent of maleic acid with one equivalent of Mg(OH)₂. The magnesium salt of nitrilotriacetic acid results from the stoechiometric reaction of two equivalents of nitrilotriacetic acid (NTA) with three equivalents of Mg(OH)₂ and can be written NTA₂Mg₃. The magnesium salt of methylglycine diacetic acid results from the stoechiometric reaction of two equivalents of methylglycine diacetic acid (MGDA) with three equivalents of Mg(OH)₂ and can be written MGDA₂Mg₃.

An example of a partial magnesium salt of the carboxylic acids of the present invention is the compound Citrate₂Mg_(1.5) resulting from the reaction of two equivalents of citric acid with 1.5 equivalents of Mg(OH)₂, said formula being actually an average one and the resulting product being in reality a mixture of compounds, i.e. a mixture of the acid and of the mono-, di- and tri-saponified molecules.

It has to be noted that, depending on the nature of the carboxylic acids according to the present invention and on their degree of saponification with magnesium (Mg²⁺), the properties of the resulting product will be different. For example, the stoechiometric compound Citrate₂Mg₃ has a solubility in water of maximum 10% by weight at room temperature, while the solubility of the partial magnesium salt Citrate₂Mg_(1.5) is of at least 30% by weight in water at room temperature. The solubility in water at room temperature of the stoechiometric magnesium salt of maleic acid is of maximum 20% by weight and of the stoechiometric magnesium salt of nitrilotriacetic acid (NTA₂Mg₃) is of at least 30% by weight.

The pretreatment step of the process of the present invention, especially according to the first or the second embodiment disclosed above, is typically performed at a neutral pH or at a weakly acidic pH, in particular at the natural pH of the paper pulp. In general, the pretreatment step is conducted at a pH equal to or above 4, particularly at a pH equal to or above 5. The pH during the pretreatment step is usually equal to or below 8, in many cases equal to or below 7, especially equal to or below 6.5, values equal to or below 6 being common. Suitable ranges for the pH during the pretreatment step are for example from 4 to 8, preferably from 5 to 7, and even more preferably from 5 to 6.

If necessary, the pH of the paper pulp may be adjusted to preferred values by means of pH modifying compounds. Examples of such compounds are sodium hydroxide or suitable alternative bases and sulfuric acid or chlorhydric acid or suitable alternative acids.

In the process of the present invention, the pretreatment step is usually carried out in the presence of water in order to reach a final consistency of at least 1% by weight of dry paper pulp. By final consistency is meant the amount of paper pulp present into the paper pulp slurry after the addition of the chelating agents needed for the pretreatment step. The final consistency of the paper pulp is usually less than or equal to 20% by weight, particularly less than or equal to 15%, more particularly less than or equal to 10%. In the majority of the cases, a final consistency of 1 to 20% by weight of dry pulp will be suitable, preferably from 3 to 15%, especially from 5 to 10%.

In the process of the invention, the pretreatment step is usually carried out at the temperature generated by the mechanical treatment of the wood. The temperature can vary widely and is usually of at least 40° C. The temperature is generally less than or equal to 95° C.

The duration of the pretreatment step is usually from 5 to 180 minutes, varying from mill to mill. In general, good results are obtained provided that the duration of the pretreatment step is of at least 5 minutes. Preferably, the duration of the pretreatment step is equal to or higher than 30 minutes.

In the process of the invention, it is recommended to have a good mixing of the paper pulp with the chelating agents during the pretreatment step. This can be done by introducing the chelating agent during via the pump circulating the pulp through the paper mill.

According to the present invention, the pretreatment step may be applied to a mechanical paper pulp, a chemical pulp or a waste paper pulp, preferably to a mechanical paper pulp. By mechanical paper pulp are meant paper pulps obtained by mechanical treatment. Examples of such paper pulps are pressure groundwood (PGW), stone groundwood (SGW), thermomechanical pulp (TMP), refiner mechanical pulp (RMP), chemithermomechanical pulp (CTMP) and alkaline peroxide mechanical pulp (APMP or APP).

In a further embodiment, the present invention relates to a process for the bleaching of paper pulp with one or more peroxide oxidizing agent, comprising the steps of

-   (a) treating the paper pulp, in a pretreatment step, with one or     more chelating agents chosen from the group consisting of     -   (i) carboxylic acids or their salts, except their magnesium         salt, substituted by at least one electrodonating group or         comprising at least one carbon-carbon double bond, in the         presence of at least one inorganic magnesium derivative, and     -   (ii) carboxylic acids at least partially saponified with Mg²⁺         and substituted by at least one electrodonating group or         comprising at least one carbon-carbon double bond. -   (b) pressing the mixture resulting from the pretreatment step (a)     until a consistency of from 25 to 50% by weight of dry pulp is     reached, preferably from 30 to 45%, and -   (c) bleaching the pressed mixture resulting from step (b) with a     peroxide oxidizing agent in the presence of a base and optionally in     the presence of a stabilizer.

Thus, the present invention also relates to a process for the bleaching of paper pulp with one or more peroxide oxidizing agent in which the pulp is first treated in a pretreatment step as described above, and comprising the additional steps of:

-   (a) pressing the mixture resulting from the pretreatment step until     a consistency of from 25 to 50% by weight of dry pulp is reached,     preferably from 30 to 45%, and -   (b) bleaching the pressed mixture resulting from step (a) with a     peroxide oxidizing agent.

Preferably, in the process of the present invention, the paper pulp is submitted to only one pretreatment step before the bleaching with one or more peroxide oxidizing agent.

A washing step may also be conducted between the pretreatment step and the bleaching step. Other steps, such as additional bleaching steps, can also be added to the process. Nevertheless, only one pretreatment step, one pressing step, one washing step and one bleaching step with one or more peroxide oxidizing agents is usually sufficient for the bleaching of mechanical paper pulp.

In the process of the invention, the peroxide oxidizing agent is usually chosen from the group consisting of hydrogen peroxide, organic peracids such as peracetic acid and/or a combination thereof. The oxidizing agent is preferably hydrogen peroxide, which is advantageously used in an alkaline medium. Hydrogen peroxide is thus preferably used in combination with an alkaline compound, especially with at least alkaline earth hydroxide such as sodium hydroxide, potassium hydroxide, calcium hydroxide and magnesium hydroxide. Preferably, the bleaching step is conducted using hydrogen peroxide, an alkaline compound and a stabilizer, such as sodium silicate or an organic stabilizer such as a polyacrylate and/or a polyhydroxyacrylate salt.

The present invention also relates to the use as chelating agents of

-   (i) a mixture of at least one carboxylic acid or salt thereof,     except its magnesium salt, combined with at least one inorganic     magnesium derivative, or -   (ii) at least one carboxylic acid at least partially saponified with     Mg²⁺, the carboxylic acids being substituted by at least one     electrodonating group or comprising at least one carbon-carbon     double bond. Is also part of the invention, the use of said     compounds as chelating agents in the pretreatment step of a process     for the bleaching of paper pulp with one or more peroxide oxidizing     agents.

The present invention is further illustrated with some examples below, without limiting the scope thereto.

EXAMPLES

The brightness of paper pulp was measured using a Datacolor Elrepho spectrophotometer (SF450) according to ISO standard 2470.

In the examples, the percentages of chelating agents are expressed as the amount of pure product by weight of dry pulp.

Examples conducted in the absence of a chelating agent are the result of a pretreatment step performed with demineralised water only. The bleaching step was conducted in the same conditions as the other examples of the series.

In the examples below, DTPA means diethylene triamine pentaacetic acid. DTPA was used as a commercial solution of 40% DTPA by weight of water.

Citrate₂Mg_(1.5) is a partially saponified magnesium salt of citric acid. This product results from the reaction of two equivalents of citric acid with 1.5 equivalents of Mg(OH)₂, said formula being actually an average one and the resulting product being in reality a mixture of compounds.

Maleic Mg means the magnesium salt of maleic acid. It results from the stoechiometric reaction of 1 equivalent of maleic acid with one equivalent of Mg(OH)₂.

NTA means nitrilotriacetic acid. NTA is commercially available, for instance as TRILON AS® from BASF. NTA Na₃ means the tri sodium salt of nitrilotriacetic acid. This product is commercially available, for example as TRILON A® sold by BASF. NTA₂Mg₃ means the magnesium salt of the nitrilotriacetic acid (NTA). NTA₂Mg₃ was prepared by reacting 2 equivalents of NTA (commercially available as TRILON AS® from BASF) with 3 equivalents of Mg(OH)₂. NTA, NTA Na₃ and NTA₂Mg₃ were used as aqueous solutions (40 wt %).

MGDA means methylglycine diacetic acid. MGDA Na₃ means the trisodium salt of methylglycine diacetic acid. This product is commercially available, for example as TRILON M® from BASF. MGDA and MGDA Na₃ were used as aqueous solutions (40 wt %).

In the examples below, MGDA Na₃/NTA means a mixture of the trisodium salt of methylglycine diacetic acid (MGDA Na₃) and of nitrilotriacetic acid (NTA). This product was prepared by adding 1.6 g of NTA stepwise (by 0.4 g) to 10 g of MGDA Na₃. The resulting mixture was diluted with water in order to obtain a concentration of active product of 40% by weight. This solution contained 72% by weight of MGDA Na₃ and 28% by weight of NTA, both being partially saponified and present under the form of their partial sodium salt. The pH of this solution was about 5.0.

Examples 1 to 11

A sample of a refiner mechanical pulp (RMP) was pretreated with the chelating agents summarized in Table I below.

The pretreatment step was carried out at 60° C. during 30 minutes on pulp samples at a final consistency in water of 10 wt %, at the natural pH of the paper pulp. After said pretreatment, the pulp was pressed up to a consistency of 30 wt %.

The resulting paper pulp was then subsequently bleached at a final consistency of 15 wt % and at a temperature of 70° C. during 120 minutes in the presence of 5.0% of hydrogen peroxide, 3.1% of caustic soda and 3.5% of sodium silicate, all these percentages being expressed by weight of dry pulp.

The brightness of the resulting bleached paper pulp and the hydrogen peroxide consumption are summarized in Table I below.

TABLE I Ex- H₂O₂ am- Amount Brightness consumption ples Chelating agent (%) (°ISO) (%) 1 — — 71.7 93.4 2 DTPA 0.16 74.5 66.0 3 Mg²⁺ 0.06 72.6 71.6 4 Citric acid + 0.2 + 73.5 72.6 Mg²⁺ (MgSO₄•7H₂O) 0.06 5 Citric acid + 0.4 + 73.9 70.0 Mg²⁺ (MgSO₄•7H₂O) 0.06 6 Citric acid + 0.6 + 74.4 69.2 Mg²⁺ (MgSO₄•7H₂O) 0.06 7 Citric acid + 0.8 + 74.5 70.0 Mg²⁺ (MgSO₄•7H₂O) 0.06 8 Citrate₂Mg_(1.5) 0.69 74.0 73.8 9 Maleic Mg 0.31 73.5 75.4 10 MGDA Na₃ 0.16 73.5 75.0 11 MGDA Na₃ + 0.16 + 74.4 70.0 Mg²⁺ (MgSO₄•7H₂O) 0.06

The amount of 0.69% of Citrate₂Mg_(1.5) by weight of dry pulp corresponds to an amount of 0.06% of Mg²⁺ and of 0.63% of citric acid by weight of dry pulp. The amount of 0.31% of the magnesium salt of maleic acid (Maleic Mg) by weight of dry pulp corresponds to an amount of 0.06% of Mg²⁺ and of 0.19% of maleic acid by weight of dry pulp.

Examples 12 to 27

A sample of a thermomechanical pulp (TMP) was pretreated with the chelating agents summarized in Table II below.

The pretreatment step was carried out in the same conditions as in previous examples.

The resulting paper pulp was then subsequently bleached at a final consistency of 15 wt % and at a temperature of 80° C. during 60 minutes in the presence of 3.9% of hydrogen peroxide, 1.9% of caustic soda and 1.5% of sodium silicate, all these percentages being expressed by weight of dry pulp.

The brightness of the resulting bleached paper pulp and the hydrogen peroxide consumption are summarized in Table II below.

TABLE II Bright- H₂O₂ Exam- Amount ness consumption ples Chelating agent (%) (°ISO) (%) 12 — — 58.2 98.7 13 Mg²⁺ 0.04 66.5 98.0 14 DTPA 0.16 73.1 61.6 15 DTPA 0.24 74.5 54.0 16 DTPA + 0.16 + 74.1 58.5 Mg²⁺ (MgSO₄•7H₂O) 0.04 17 Citric acid 0.6 64.7 97.9 18 Citric acid + 0.6 + 70.2 76.1 Mg²⁺ (MgSO₄•7H₂O) 0.05 19 Citrate₂Mg_(1.5) 0.5 70.3 82.0 20 Citrate₂Mg_(1.5) 0.75 71.6 75.1 21 MGDA Na₃ 0.32 63.9 73.4 22 MGDA Na₃ + 0.32 + 74.1 55.7 Mg²⁺ (MgSO₄•7H₂O) 0.05 23 MGDA Na₃ + 0.32 + 74.1 74.7 Mg²⁺ (MgSO₄•7H₂O) 0.06 24 MGDA Na₃ + 0.24 + 73.2 79.5 Mg²⁺ (MgSO₄•7H₂O) 0.06 25 MGDA Na₃/NTA + 0.32 + 72.9 78.6 Mg²⁺ (MgSO₄•7H₂O) 0.06 Final pH at pretreatment step = 4.5 26 MGDA Na₃/NTA + 0.32 + 74.5 62.9 Mg²⁺ (MgSO₄•7H₂O) 0.06 pH maintained between 6 and 6.7 27 MGDA Na₃/NTA + 0.24 + 74.3 66.6 Mg²⁺ (MgSO₄•7H₂O) 0.06 pH maintained between 6 and 6.7

The amount of 0.5% of Citrate₂Mg_(1.5) by weight of dry pulp corresponds to an amount of 0.04% of Mg²⁺ and of 0.46% of citric acid by weight of dry pulp. The amount of 0.75% of Citrate₂Mg_(1.5) by weight of dry pulp corresponds to an amount of 0.06% of Mg²⁺ and of 0.69% of citric acid by weight of dry pulp.

Examples 28 to 34

A sample of a chemithermomechanical pulp (CTMP) was pretreated with the chelating agents summarized in Table III below.

The pretreatment step was carried out in the same conditions as in the previous examples.

The resulting paper pulp was then subsequently bleached at a final consistency of 15 wt % and at a temperature of 65° C. during 120 minutes in the presence of 2.5% of hydrogen peroxide, 1.7% of caustic soda and 2.3% of sodium silicate, all these percentages being expressed by weight of dry pulp.

The brightness of the resulting bleached paper pulp and the hydrogen peroxide consumption are summarized in Table III below.

TABLE III H₂O₂ Exam- Amount Brightness consumption ples Chelating agent (%) (°ISO) (%) 28 — — 64.4 97.6 29 DTPA 0.12 73.4 64.1 30 Mg²⁺ 0.06 71.2 88.6 31 Citrate₂Mg_(1.5) 0.69 72.5 72.6 32 Maleic Mg 0.31 70.5 90.4 33 NTA Na₃ 0.24 73.4 76.1 34 NTA₂Mg₃ 0.28 73.4 68.8

The amount of 0.69% of Citrate₂Mg_(1.5) by weight of dry pulp corresponds to an amount of 0.06% of Mg²⁺ and of 0.63% of citric acid by weight of dry pulp. The amount of 0.31% of the magnesium salt of maleic acid (Maleic Mg) by weight of dry pulp corresponds to an amount of 0.06% of Mg²⁺ and of 0.19% of maleic acid by weight of dry pulp. The amount of 0.28% of NTA₂Mg₃ by weight of dry pulp corresponds to an amount of 0.04% of Mg²⁺ and of 0.24% of NTA by weight of dry pulp.

Examples 35 to 41

A sample of a stone ground wood pulp (SGW) was pretreated with the chelating agents summarized in Table IV below.

The pretreatment step was carried out in the same conditions as in the previous examples.

The resulting paper pulp was then subsequently bleached at a final consistency of 15 wt % and at a temperature of 80° C. during 180 minutes in the presence of 3.5% of hydrogen peroxide, 1.17% of Kaliol (magnesium hydroxide suspension) and 0.2% of DTPA, all these percentages being expressed by weight of dry pulp.

The brightness of the resulting bleached paper pulp and the hydrogen peroxide consumption are summarized in Table IV below.

TABLE IV Bright- H₂O₂ Exam- Amount ness consumption ples Chelating agent (%) (°ISO) (%) 35 — — 71.8 86.6 36 DTPA 0.16 75.8 43.8 37 NTA Na₃ 0.5 75.0 54.3 38 NTA Na₃ 0.24 74.0 60.0 39 NTA Na₃ + 0.24 + 75.3 55.7 Mg²⁺ (MgSO₄•7H₂O) 0.06 40 NTA₂Mg₃ 0.24 75.4 52.0 41 MGDA Na₃ + 0.16 + 75.0 61.8 Mg²⁺ (MgSO₄•7H₂O) 0.08

The amount of 0.24% of NTA₂Mg₃ by weight of dry pulp corresponds to an amount of 0.04% of Mg²⁺ and of 0.28% of NTANa₃ by weight of dry pulp.

Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it might render a term unclear, the present description shall take precedence. 

1. A process for the bleaching of paper pulp with one or more peroxide oxidizing agents, comprising first treating the pulp in a pretreatment step with one or more chelating agents chosen selected from the group consisting of (i) carboxylic acids or their salts, except their magnesium salt, substituted by at least one electrodonating group or comprising at least one carbon-carbon double bond, in the presence of at least one inorganic magnesium derivative, and (ii) carboxylic acids at least partially saponified with Mg²⁺ and substituted by at least one electrodonating group or comprising at least one carbon-carbon double bond.
 2. The process according to claim 1, wherein the electrodonating group is selected from the group consisting of amines, alcohols, thiols, O-alkyl, and S-alkyl.
 3. The process according to claim 1, wherein the carboxylic acid has the following general formula HOOC—C_(n)H_(m)(OR¹)_(p)(SR²)_(q)(NR³R⁴)_(r)(COOH)_(s)—R′ wherein C_(n)H_(m) is linear or branched and saturated or unsaturated; n is an integer from 1 to 8; m is an integer from 0 to 2n; p is an integer from 0 to n; q is an integer from 0 to n; r is an integer from 0 to n; s is an integer from 0 to n; p+q+r+s is an integer from 0 to 2n, and if p+q+r+s is 0, m is ≦2n−2; R′ is selected from the group consisting of —H, —CH₂OH, and —COOH; and R¹, R², R³, R⁴ are the same or different and are selected from the group consisting of —H and linear or branched groups of formula —C_(n′)H_(m′)—R″, wherein n′ is an integer from 1 to 8, m′ is an integer from 0 to 2n, and R″ is selected from the group consisting of —H, —CH₂OH, and —COOH.
 4. The process according to claim 1, wherein the carboxylic acid is selected from the group consisting of lactic acid, citric acid, tartaric acid, glucaric acid, maleic acid, nitrilotriacetic acid, methylglycine diacetic acid, their salts, and mixtures thereof.
 5. The process according to claim 1, wherein the inorganic magnesium derivative is selected from the group consisting of magnesium oxide, magnesium hydroxide, and magnesium sulfate.
 6. The process according to claim 1, wherein the at least one chelating agent is the carboxylic acid at least partially saponified with Mg²⁺ (ii).
 7. The process according to claim 1, wherein the pretreatment step is performed at the natural pH of the paper pulp.
 8. The process according to claim 1, wherein the concentration of the inorganic magnesium derivative, expressed as Mg²⁺ ions, is less than or equal to 1% by weight of the dry paper pulp and is more than or equal to 0.01% by weight of the dry paper pulp.
 9. The process according to claim 1, wherein the concentration of the carboxylic acid or its salt (i) or of the carboxylic acid at least partially saponified with Mg²⁺ (ii) is less than or equal to 1% by weight of the dry pulp and is more than or equal to 0.2% by weight of the dry pulp.
 10. The process according to claim 1, being applied to a mechanical paper pulp.
 11. The process according to claim 1, wherein the pretreatment step is carried out in the presence of water in order to reach a consistency of 1 to 20% by weight of dry pulp.
 12. The process according to claim 1, comprising the additional steps of: (a) pressing the mixture resulting from the pretreatment step until a consistency of from 25 to 50% by weight of dry pulp is reached, and (b) bleaching the pressed mixture resulting from step (a) with a peroxide oxidizing agent.
 13. The process according to claim 1, wherein the peroxide oxidizing agent is selected from the group consisting of hydrogen peroxide, organic peracids, and a combination thereof.
 14. A method of use as chelating agents of (i) a mixture of at least one carboxylic acid or salt thereof, except its magnesium salt, combined with at least one inorganic magnesium derivative, or (ii) at least one carboxylic acid at least partially saponified with Mg²⁺, the carboxylic acids being substituted by at least one electrodonating group or comprising at least one carbon-carbon double bond.
 15. The method according to claim 14, being carried out in a pretreatment step of a process for the bleaching of paper pulp with one or more peroxide oxidizing agents. 